1) Seadragon- Impossible to legally own in the US. Even if you lived in a different country, and could pass through the proper CITES requirements, do the pounds of paperwork, actually find a reputable seller...yatta yatta yatta...then you would need a large cold tank, and most of the keepers of seadragons agree that it should be round.
2)Cuttlefish- Yes, you can keep these, but the most common species found in the US (S. officinallis) gets large, and need cold water. But if you look around enough, then you can find S. bandensis, a much smaller species, that does not need the really cold water. But be foreworned, they do not live long.